1. Field of the Invention
The present invention relates to the field of electronic data recording and playback devices, and more specifically to a magnetic data storage and retrieval system.
2. Prior Art
Throughout the history of systems used for recording and retrieval of electronic digital data, one major difficulty has continued to plague the designers and operators of such systems. This difficulty has been the inability to develop an inexpensive and reliable recording and playback system which can record data at a constant density on the recording media and retrieve the recorded data at a rate equal to the rate at which it was recorded. Such constant density and correct playback speed are needed to prevent errors in both the recording and retrieval. That is, if digital data is recorded at one speed and read at a different speed, the data would no longer be synchronous with the device utilizing the data, such as a digital computer. This asynchronous condition results in the computer accepting one bit of data when it should be accepting a different bit of data which was recorded either earlier or later. Traditionally, the requirements of constant data density and consistent recording and playback speeds have been met by controlling the motor speed of the associated tape drive mechanism. Such control is typically maintained by an electronic feedback means which monitors the tape drive speed by a motor speed indicator and adjusts the speed based on the monitored information. Thus, in the feedback method of control, an adjustment capability is present in the feedback loop so that the speed of the tape can be accurately adjusted.
However, the prior art devices are subject to several disadvantages. One disadvantage is that the feedback systems are not stable over a period of time so that the motor speed can vary. Such variance can only be eliminated by alignment procedures performed in the field which require the skill of a well-trained technician. The variation in speed can result from a variety of factors including the effects of motor wear and aging, the effects of drive wheel wear and aging, and the variance of electronic components with use. In many prior art devices, this alignment procedure must be performed as often as every two months, causing loss of the use of the recording system until the alignment can be completed.
A second disadvantage of the prior art feedback systems is that they require costly and accurate electric motors with precision machined drive wheels. Such motors must be capable of consistently responding to the changes in speed mandated by the feedback loop. This consistent change in motor speed must then be consistently translated into an accurate tape drive speed by a precision machined drive wheel. If either the motor or the drive wheel fails to accurately respond, the tape drive speed can vary resulting in data errors. A third disadvantage of the prior art recording devices is that the feedback systems used to maintain the motor speed must be complex and therefore costly in order to maintain a constant speed. This problem is further compounded by the necessity in modern computers of fast access time to particular locations on a reel of magnetic tape. Fast access time requires high acceleration of the tape drive unit. However, high acceleration necessitates an even more complex design of the feedback loop so as to prevent instabilities and the corresponding failures of the system.
A second major problem associated with systems used for recording electronic digital data has been the difficulty in indicating to the device using the recording system where various areas are located on the reel of magnetic tape. For example, it is important to indicate to a computer utilizing a tape recording system for data storage where the beginning and end of the tape are located and also where to load data onto the tape. The prior art devices have provided this information to the computer by utilizing magnetic tape which has a small hole through the tape near its beginning and end or has reflective tape portions, and light sensors adapted to sense the passage of the holes or reflected light. However, such sensors are very sensitive to positional changes and thus require frequent alignment procedures.
Accordingly, it is a general object of the present invention to provide an improved data storage and retrieval system.
It is another object of the present invention to provide an improved data storage and retrieval system which is relatively insensitive to variations in the motor speed of the tape drive mechanism.
It is another object of the present invention to provide a relatively inexpensive yet accurate data storage and retrieval system.
It is yet another object of the present invention to provide an improved data storage and retrieval system which is capable of denoting various areas located on a reel of magnetic tape.